Users demand provision of voice connections over cellular networks. In existing GSM and UMTS networks, these can be provided using dedicated Circuit Switched (CS) technology over the Radio Access Network (RAN). The third generation partnership project (3GPP) has developed standards for a cellular communication system, referred to as long term evolution (LTE). LTE does not support CS technology. Instead, it is expected that an IP Multimedia Subsystem (IMS) will be used for voice connections in LTE using Voice over Internet Protocol (VoIP).
Mobile stations, which are usually termed user equipment (UE), can operate at any one time with one of the: GSM EDGE RAN (GERAN); the UMTS Terrestrial RAN (UTRAN); and the LTE Enhanced-UTRAN (E-UTRAN). A GSM/GPRS network is not typically able to provide the bandwidth, Quality of Service (QoS) and handover to provide a VoIP call using an IMS. Some UMTS systems and a typical LTE system can provide this service, however. Consequently, a UE can make voice connections using CS technology over GERAN or UTRAN systems or use VoIP over E-UTRAN and some UTRAN system.
The IMS in the core network of the cellular system cannot see the underlying network beneath the Internet Protocol (IP) layer. This has some advantages, but adds a difficulty when routing incoming voice calls or sessions to a UE. The RAN is invisible to the IMS, since when a UE changes RAN, its IP address remains the same. If the UE is being served by a RAN without VoIP capability, the voice call needs to be routed in some other way, for example via CS infrastructure. When the UE then moves to LTE, which does not support CS technology, the voice call should be re-routed over VoIP.
Existing mechanisms to route a mobile-terminating voice connection first try VoIP over IMS and then drop back to CS, if the VoIP call cannot be supported, for example due to a lack of bandwidth. However, this drop back method is problematic, in that it adds to call set-up delay for all incoming calls. Moreover in the worst case, a time-out is reached by the calling party waiting for a connection and the call fails. Also, if a UE moves between a RAN with VoIP capability and one without this capability, the call can be lost because of the time needed to re-route the call. It is therefore desirable to route voice connections in a more efficient way over both RANs that are VoIP-capable and RANs that are VoIP-incapable.